The LHC / LC Study Group and the Snowmass Points and Slopes

8 Pages·0.13 MB·English

by  Georg Weiglein

DCPT/02/152
IPPP/02/76
hep-ph/0301111
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The LHC / LC Study Group
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2 and the Snowmass Points and Slopes∗

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J
G. Weiglein†
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v Institute for Particle Physics Phenomenology,
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University of Durham, Durham DH1 3LE, U.K.
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h Abstract
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p The “LHC / LC Study Group” investigates how analyses at the LHC could
e profit from results obtained at a future Linear Collider and vice versa. Some of
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the activities of this recently formed working group are briefly summarised. The
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v LHC / LC Study Group home page is www.ippp.dur.ac.uk/∼georg/lhclc. The
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X “Snowmass Points and Slopes” (SPS) are a set of benchmark points and parameter
r linesintheMSSMparameterspacecorrespondingtodifferentscenariosinthesearch
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for Supersymmetry at present and future experiments. This set of benchmarks was
agreed upon at the 2001 “Snowmass Workshop on the Future of Particle Physics”
as a consensus based on different existing proposals. Further information about the
SPS can be found under www.ippp.dur.ac.uk/∼georg/sps.
∗Talk given by G. Weiglein at SUSY02, June 2002,DESY, Germany
†email: [email protected]
1 The LHC / LC Study Group
The aim of the LHC / LC Study Group is to investigate a possible cross-talk between the
LHC anda future Linear Collider (LC) and to study inhow far analyses carried out at one
of the machines could profit from results obtained at the other machine. Mutual benefits
could occur both at the level of a combined interpretation of Hadron Collider and Linear
Collider data and at the level of combined analyses of the data, where results obtained at
one machine could directly influence the way analyses are carried out the other machine.
Topics under study comprise the physics of weak and strong electroweak symmetry
breaking, Supersymmetric models, new gauge theories, models with extra dimensions,
and electroweak and QCD precision physics. For these studies it is assumed that the LC
comes into operation about half a decade after the start of the LHC. During simultaneous
running of both machines there is obviously the highest flexibility for adapting analyses
carried out at one machine according to the results obtained at the other machine. The
LC results could in this context also serve as an input for a second phase of LHC running
concerning different possible upgrade options.
To mention just one example of studies carried out in the working group, the deter-
mination of masses of Supersymmetric particles at the LHC could profit from the precise
measurement of the lightest Supersymmetric particle (LSP) at the LC. As a trivial il-
lustration of the possible benefits of a precise measurement of the LSP mass at the LC,
Fig. 1 shows the relative accuracy of the determination of the mass of the next-to-lightest
neutralino at the LHC, which is highly correlated with the accuracy of the LSP mass
determination [1]. The prospective accuracy of the LSP mass measurement at the LC is
indicated by a narrow band, which obviously leads to a drastic improvement of the mass
determination of the next-to-lightest neutralino at the LHC.
The LHC / LC Study Group has been forming over the last months and is still fur-
ther expanding. Currently there are about 140 working group members (members of
ATLAS, CMS, LC working groups, a Tevatron contact person, and theorists). Further
information about the working group can be obtained from [email protected],
[email protected], [email protected] or from the LHC / LC Study Group
web page www.ippp.dur.ac.uk/∼georg/lhclc.
2 The Snowmass Points and Slopes (SPS)
In the unconstrained version of the Minimal Supersymmetric extension of the Standard
Model (MSSM) no particular Supersymmetry (SUSY) breaking mechanism is assumed,
but rather a parameterisation of all possible soft SUSY breaking terms is used. This
leads to more than a hundred parameters (masses, mixing angles, phases) in this model
in addition to the ones of the Standard Model. For performing detailed simulations of
experimental signatures within detectors of high-energy physics experiments it is clearly
not practicable to scan over a multi-dimensional parameter space. One thus often con-
centrates on certain “typical” benchmark scenarios.
The Snowmass Points and Slopes (SPS) [2] are a set of benchmark scenarios in the
MSSM parameter space which was agreed upon at the 2001 “Snowmass Workshop on the
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Figure 1: The relative accuracy in the determination of the masses of the lightest and the
next-to-lightest neutralino at the LHC, taken from Ref. [1]. The narrow band overlayed
in the plot shows the improvement in the mass determination of the next-to-lightest
neutralino at the LHC if the measurement of the LSP mass at the LC is used as input.
Future of Particle Physics” as a consensus based on different existing proposals [3]. The
SPS consist of model lines (“slopes”), i.e. continuous sets of parameters depending on
one dimensionful parameter and specific benchmark points, where each model line goes
through one of the benchmark points. The SPS should be regarded as a recommendation
for future studies of SUSY phenomenology, but of course are not meant as an exclusive
and for all purposes sufficient collection of SUSY models. They mainly focus on “typical”
scenarios within the three currently most prominent SUSY-breaking mechanisms, i.e.
minimal supergravity (mSUGRA) [4], gauge-mediated SUSY breaking (GMSB) [5], and
anomaly-mediated SUSY breaking (AMSB) [6]. Furthermore they contain examples of
“more extreme” scenarios, e.g. a “focus point” scenario [7] with a rather heavy SUSY
spectrum, indicating in this way different possibilities for SUSY phenomenology that can
be realised within the most commonly used SUSY breaking scenarios.
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The SPS comprise ten benchmark points, from which six correspond to an mSUGRA
scenario, one is an mSUGRA-like scenario with non-unified gaugino masses, two refer to
the GMSB scenario, and one to the AMSB scenario. Seven of these benchmark points
are attached to model lines, while the remaining three are supplied as isolated points.
In studying the benchmark scenarios the model lines should prove useful in performing
more general analyses of typical SUSY signatures, while the specific points indicated on
the lines are proposed to be chosen as the first sample points for very detailed (and thus
time-consuming) analyses. The concept of a model line means of course that more than
just one point should be studied on each line. Results along the model lines can often
then be roughly estimated by interpolation.
The SUSY-breaking scenarios mentioned above are characterised by a few input pa-
rameters (in the mSUGRA scenario, for instance, these are the scalar mass parameter m ,
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the gaugino mass parameter m , the trilinear coupling A , the ratio of the Higgs vacuum
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expectation values, tanβ, and the sign of the Supersymmetric Higgs mass parameter, µ).
The mass spectra ofthe SUSY particles inthe mSUGRA, GMSBand AMSB scenarios are
obtained via renormalisation group running from the scale of the high-energy parameters
of the SUSY-breaking scenario to the weak scale. The low-energy parameters obtained in
this way are then used as input for calculating the predictions for the production cross
sections and for the decay branching ratios of the SUSY particles.
An important aspect in the philosophy behind the benchmark scenarios is that the
low-energy MSSM parameters are defined to be the actual benchmark rather than the
high-energy input parameters m , m , etc. A specification of the benchmark scenarios
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in terms of the latter parameters is merely understood as an abbreviation for the low-
energy phenomenology (it also depends on the particular program used for relating the
high-energy input parameters to the low-energy MSSM parameters).
While certain sets of low-energy MSSM parameters have been fixed as benchmarks
in the SPS by definition (which in principle could have been done without resorting at
all to scenarios like mSUGRA, GMSB and AMSB), the evaluation of the mass spectra
and decay branching ratios from the MSSM benchmark parameters should be carried out
with the tools and at the level of sophistication being most appropriate for the particular
application one is interested in. If detailed comparisons between different experiments or
different colliders are carried out, it would clearly be advantageous to use the same results
for the mass spectra and the branching ratios.
The main qualitative difference between the SPS (and also the recent proposals for
post-LEP benchmarks in Ref. [3]) and the benchmarks used previously for investigating
SUSY searches at the LHC, the Tevatron and a future Linear Collider is that scenarios
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with small values of tanβ, i.e. tanβ ∼ 3, are disfavoured as a result of the Higgs exclusion
bounds obtained at LEP. Consequently, there is more focus now on scenarios with larger
values of tanβ than in previous studies. Concerning the SUSY phenomenology, interme-
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diate and large values of tanβ, tanβ ∼ 5, have the important consequence that there is
in general a non-negligible mixing between the two staus (and an even more pronounced
mixinginthesbottomsector), leadingtoasignificant masssplittingbetween thetwo staus
so that the lighter stau becomes the lightest slepton. Neutralinos and charginos therefore
decay predominantly into staus and taus, which is experimentally more challenging than
the dilepton signal resulting for instance from the decay of the second lightest neutralino
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into the lightest neutralino and a pair of leptons of the first or the second generation.
Large values of tanβ can furthermore have important consequences for the phe-
nomenology in the Higgs sector, as the couplings of the heavy Higgs bosons H, A to
down-type fermions are in general enhanced. For sizable values of µ and m the hb¯b cou-
g˜
pling receives large radiative corrections from gluino loop corrections, which in particular
affect the branching ratio BR(h → τ+τ−).
The main features of the SPS benchmarks are listed in the following:
SPS 1: “typical” mSUGRA scenario
This scenario consists of a “typical” mSUGRA point with an intermediate value
of tanβ and a model line attached to it (SPS 1a) and of a “typical” mSUGRA
point with relatively high tanβ (SPS 1b). The two-points lie in the “bulk” of
the cosmological region where the lightest SUSY particle (LSP) gives rise to an
acceptable dark matter density. For the collider phenomenology in particular the
τ-rich neutralino and chargino decays are important.
SPS 2: “focus point” scenario in mSUGRA
The benchmark point chosen for SPS 2 lies in the “focus point” region, where a too
large relic abundance is avoided by an enhanced annihilation cross section of the
LSP due to a sizable higgsino component. This scenario features relatively heavy
squarks and sleptons, while the charginos and the neutralinos are fairly light and
the gluino is lighter than the squarks.
SPS 3: model line into “coannihilation region” in mSUGRA
The model line of this scenario is directed into the “coannihilation region”, where
a sufficiently low relic abundance can arise from a rapid coannihilation between
the LSP and the (almost mass degenerate) next-to-lightest SUSY particle (NLSP),
which is usually the lighter τ˜. Accordingly, an important feature in the collider
phenomenology of this scenario is the very small slepton–neutralino mass difference.
SPS 4: mSUGRA scenario with large tanβ
Thelargevalueoftanβ inthisscenariohasanimportantimpactonthephenomenol-
ogy in the Higgs sector. The couplings of A,H to b¯b and τ+τ− as well as the H±t¯b
couplings are significantly enhanced in this scenario, resulting in particular in large
associated production cross sections for the heavy Higgs bosons.
SPS 5: mSUGRA scenario with relatively light scalar top quark
This scenario is characterised by a large negative value of A , which allows consis-
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tency of the relatively low value of tanβ with the constraints from the Higgs search
at LEP, see Ref. [8].
SPS 6: mSUGRA-like scenario with non-unified gaugino masses
In this scenario, the bino mass parameter M is larger than in the usual mSUGRA
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models by a factor of 1.6. While a bino-like neutralino is still the LSP, the mass
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differencebetweenthelightestcharginoandthelightesttwoneutralinosandtheslep-
tons is significantly reduced compared to the typical mSUGRA case. Neutralino,
chargino and slepton decays will feature less-energetic jets and leptons as a conse-
quence.
SPS 7: GMSB scenario with τ˜ NLSP
The NLSP in this GMSB scenario is the lighter stau, with allowed three body
decays of right-handed selectrons and smuons into it. The decay of the NLSP into
the Gravitino and the τ in this scenario can be chosen to be prompt, delayed or
quasi-stable.
SPS 8: GMSB scenario with neutralino NLSP
The NLSP in this scenario is the lightest neutralino. The second lightest neutralino
has a significant branching ratio into h when kinematically allowed. The decay of
the NLSP into the Gravitino (and a photon or a Z boson) in this scenario can be
chosen to be prompt, delayed or quasi-stable.
SPS 9: AMSB scenario
This scenario features a very small neutralino–chargino mass difference, which is
typical for AMSB scenarios. Accordingly, the LSP is a neutral wino and the NLSP
a nearly degenerate charged wino. The NLSP decays to the LSP and a soft pion
with a macroscopic decay length, as much as 10 cm.
As an example, below the benchmark values (i.e. the low-energy MSSM parameters)
are given for the benchmark point of SPS 1a. All mass parameters are given in GeV. The
value of the top-quark mass for all SPS benchmarks is chosen to be m = 175 GeV.
t
AllmassparametersforthebenchmarkpointofSPS1aaretobeunderstoodasdefined
in the DR scheme at the scale Q = 454.7 GeV.
The gluino mass M , the Supersymmetric Higgs mass parameter µ, the mass of the CP-
g˜
oddHiggsbosonM , theratioofthevacuumexpectation valuesofthetwo Higgsdoublets
A
tanβ, andthe electroweak gaugino massparameters M andM have thefollowing values:
1 2
M = 595.2, µ = 352.4, M = 393.6, tanβ = 10, M = 99.1, M = 192.7. (1)
g˜ A 1 2
ThesoftSUSY-breaking parametersinthediagonalentriesofthesquarkandsleptonmass
matrices have been chosen to be the same for the first and second generation. They have
the following values (these parameters are approximately equal to the sfermion masses;
the off-diagonal entries have been neglected for the first two generations; the index i in
M refers to the generation):
q˜iL
M = M = 539.9, M = 519.5, M = 521.7, M = 196.6, M = 136.2.
q˜1L q˜2L d˜R u˜R e˜L e˜R
(2)
The soft SUSY-breaking parameters in the diagonal entries of the squark and slepton
mass matrices of the third generation have the following values,
M = 495.9, M = 516.9, M = 424.8, M = 195.8, M = 133.6, (3)
q˜3L ˜bR t˜R τ˜L τ˜R
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SPS 1a
800
m [GeV]
700
600 g˜
t˜2
u˜ ,d˜
u˜L,d˜R ˜b2
R L
500 ˜b1
400 H0,A0 H± χχ˜˜004 χ˜±2 t˜1
3
300
200 ˜νl˜Ll τ˜2 χ˜0 χ˜±
2 1
˜l
h0 R τ˜1
100 χ˜0
1
0
Figure 2: The SUSY particle spectrum for the benchmark point corresponding to SPS 1a
(see Refs. [2,10]).
while the trilinear couplings of the third generation read
A = −510.0, A = −772.7, A = −254.2. (4)
t b τ
The corresponding SUSY particle spectrum as obtained with ISAJET 7.58 [9]is shown
in Fig. 2.
ThebenchmarkvaluesfortheotherSPScanbefoundatwww.ippp.dur.ac.uk/∼georg/sps,
see also Ref. [10].
Acknowledgements
The author thanks the members of the LHC / LC Study Group and the authors of
hep-ph/0202233 for their collaboration. I am grateful to the organisers of SUSY02 for the
invitation and the very pleasant atmosphere at the conference.
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